1. Field of the Invention
This invention relates generally to cookware and, more particularly, to cookware having a stick resistant coating applied thereto.
Heretofore, it has been common to apply a lubricative layer of PTFE (polytetrafluoroethylene) to the cooking surface of metal cookware to provide, at least initially, a relatively stick-free surface. Over time, the PTFE loses lubricity and becomes less stick-free or somewhat stick resistant. The PTFE surface is somewhat objectionable because much softer than the metal substrate and is easily scratched and otherwise marred by metal spatulas, forks and other kitchen utensils used for cooking. Once scratched or otherwise marred, the PTFE surface can be chipped off and generally loses its original utility as a stick-free surface.
2. Description of the Related Art
More recently, attempts have been made, as disclosed in UK Patent Application GB 2,277,466A to Tsai, to increase the scratch resistance of PTFE. Other work in the area aimed at improving the hardness and durability of stick resistant coatings for cookware is embodied in U.S. Pat. No. 5,447,803 to Nagaoka et al., which discloses a coating of titanium nitride to steel fry pans. The TiN coating is applied by physical vapor deposition (PVD) or by chemical vapor deposition (CVD) and then heat treated in an atmosphere consisting of oxygen and nitrogen in a controlled ratio to form a protective layer of titanium oxide on the surface of the titanium nitride layer. The titanium nitride layer is said to be hard and, in addition, produces a pleasing golden color to improve the appearance of the fry pan.
Zirconium nitride is a known ceramic coating which has been applied to enhance hardness and wear resistance to various industrial goods such as drill bits, lathe bits and injection molds.
I have discovered that zirconium nitride provides an excellent long-lasting, stick resistant coating for cookware when the substrate metal is properly selected and prepared. In addition, zirconium nitride does not require a separate treatment step to produce an oxide surface as is necessary with prior titanium nitride surfaces. Further, the zirconium nitride coating of my invention provides a pleasing golden color on the cookware which does not discolor when exposed to direct flame, which occurs with titanium nitride coatings. The zirconium nitride coating of the present invention is extremely hard and abrasion resistant, which provides a durable, long-lasting, stick resistant cooking surface, thus making it particularly suitable for restaurant and institutional kitchens. The pleasing gold tone color of the zirconium nitride coating, coupled with its stick resistant properties on the cookware of the present invention, likewise, makes it suitable for the housewares consumer market.
I have also discovered that an improved stick resistant cook surface can be provided when the bare substrate metal is provided with a bright luster finish of less than 20 microinches prior to coating with the ceramic nitride. In my co-pending application Ser. No. 09/211,749, I disclose mechanical polishing and buffing to achieve the bright luster finish. I have now discovered that a further improved stick resistant cook surface of ceramic nitride can be provided if the substrate metal surface is provided with a bright luster finish of preferably less than 10 microinches by electropolishing prior to coating.
The electropolishing step can follow a mechanical buffing step or it can be used in place of the mechanical buffing step, depending on the surface finish of the metal sheet being drawn and the final surface finish desired. As a general rule, electropolishing will reduce the surface finish of the mechanically buffed material by about 50%. The electropolishing step provides additional benefits over the mechanical buffing/polishing step of my parent application. The electropolishing step provides a smoother surface and requires no buffing abrasive compounds or lubricants which are difficult to clean and can be left in the substrate metal in mechanical polishing. These materials can cause poor coating adherence or other coating defects which adversely affect the coated cook surface. In addition, electropolishing can easily be done on the drawn cookware of various shapes. In this manner, cookware of non-round and/or deep drawn shapes can be electropolished which otherwise could not easily be polished/buffed by mechanical means. The present electropolishing step is also beneficial for round shaped cookware because of the cleaner substrate which is free from residual contaminants resulting from the mechanical polishing/buffing operation and the smoother surface finish obtained. Finally, electropolishing is economically advantageous since a plurality of cookware items may be electropolished simultaneously in the same treatment tank.
In addition, I theorize that the substrate surface after electropolishing is in a better condition than a mechanically polished/buffed surface because it has not been mechanically abraded and, thus, is better suited for subsequent ceramic nitride coating. This is due to the fact that electropolishing removes the microscopic high points of metal on the surface rather than moving or wiping the high points of the surface metal in mechanically finished surfaces.